A. Field of the Invention
This invention relates generally to fluid level detection apparatus which can be used in overfill protection devices used for protecting against overfilling of storage or transport tanks, and more particularly to fail-safe probe circuits for determining the level of fluids or other materials within the tanks.
B. Description of the Related Art
Tanks used for storing or transporting flammable fluids such as gasoline, diesel fuel and other hazardous petroleum products are often equipped with overfill protection devices to stop the filling operation when the tanks are full, and thereby prevent waste and protect the environment from spillage due to overfilling.
The tanks can be mounted on tanker trucks or located underground at service stations. Tanker trucks are typically filled with the fluids using pumping equipment at loading racks of marketing terminals, and underground storage tanks are typically gravity filled from the trucks.
An overfill protection device is used with each tank to disable the pumping equipment at the marketing terminals or to close a truck-mounted flow valve at the service station when the limit of the tank's capacity is reached.
Such an overfill protection device typically has a detection circuit and a disable circuit. The detection circuit, in turn, has a probe located within the tank which generates an electrical sensor signal that indicates when the fluid within the tank exceeds a pre-determined level. The detection circuit is connected to the disable circuit by an electrical cable so that the sensor signal is forwarded to the disable circuit. In response to the sensor signal, indicating that a particular tank is overfilled, the disable circuit operates to stop flow into that tank (for example, by disabling the pumping equipment at the loading rack or by closing the flow valve on the truck).
The detection and disable circuits are both powered from a controller circuit mounted near the tank (e.g., on the tanker truck or at the service station) and, for applications involving filling of tanks with hazardous materials, the detection circuit, the disable circuit and the controller circuit are designed to be "fail safe." In this context, "fail safe" means that no single failure of a component or wiring in the detection circuit, the disable circuit or the controller circuit can cause a condition in which the tank is overfilled or overflows.
The design and construction of fail safe power supplies for the controller circuit and for the disable circuit is well-known and the details are not of significance in understanding the present invention.
However, in order to make the detection circuit fail safe, special logic signals are used to indicate full and underfilled conditions. These special logic signals are used to prevent a short circuit or an open circuit in the output section of the detection circuit from mimicking an overfill or underfill condition. For example, if a single logic level (either a "HIGH" logic level or a "LOW" logic level were used to represent an underfilled condition, a short circuit or an open circuit would prevent the disable circuit from properly sensing the tank condition.
Accordingly, the detection circuits are conventionally designed so that the sensor signal alternates between HIGH and LOW logic states when the tank is not overfilled, whereas the sensor signal remains at a single logic state, e.g., HIGH, when the predetermined tank limit is reached or exceeded.
Known detection circuits commonly use a single pair of leads both to provide power to the detection circuit and to serve as output signal leads. One of the leads is conventionally grounded, leaving a single lead to provide power and to convey the output signal to the disable circuit. The alternating sensor signal is produced by periodically operating a switching device in the output section of the detection circuit which switching device, in turn, periodically turns "on" and places a low impedance across the detection circuit output lead which is normally maintained in a HIGH logic state by the power supply. The low impedance pulls the detection circuit output lead into a LOW logic state. Accordingly, a failure of the output switching device which produces either a permanent open or short circuit at the output switching circuit causes the alternating signal to stop thereby placing the detection circuit permanently in an "overfill" output condition and preventing an actual overfill.
While conventional alternating output detection circuits are generally suitable for their intended applications, it has been found that, under specific fault conditions, the output switching device can become shorted in such a way that it conducts current non-linearly with applied voltage. Many conventional controller circuits are designed to operate with several different types of probes, including optical probes and thermistor probes, and control the power supply voltage in accordance with the sensed current draw. In the case of a failure which produces a non-linear impedance, the detection circuit can interact with the controller to produce an alternating output which can be erroneously sensed by the disable circuit as an "alternating" voltage even when an overfill condition occurs, thereby providing false sensor signals that do not accurately indicate the level of fluid in the tank.
In addition, it is possible to connect the detection circuits to the power source in a reverse manner so that, as with a failure in the output section, a non-linear impedance across the output results which, in turn, generates an alternating output even during an overfill condition. Therefore, the conventional detection circuits are not truly "fail safe" devices.
Accordingly, it is an object of the present invention to provide a detection circuit for an overfill prevention device that is truly "fail safe".
It is another object of the present invention to provide a detection circuit for an overfill prevention device which is simple and economical in construction.
It is still another object of the present invention to provide a detection circuit for an overfill prevention device that is "fail safe" even if connected in a reverse manner to the power supply.